The present invention relates to N-substituted 3-amino-2,2-di(C-alkyl)-1,4-butyrolactones and -1,4-thiobutyrolactones, to their preparation, to the pharmaceutical compositions comprising them and to their use as stimulant of the activity of xcex3-aminobutyric acid and as medicament preferably intended for the treatment of nervous disorders.
The GABA-A Receptor
xcex3-Aminobutyric acid (or GABA (1)) is the most important inhibitory neurotransmitter of the central nervous system. It acts at the level of three separate classes of receptors known as GABA-A, GABA-B and GABA-C receptors. The GABA-A receptor, the amino acid sequence of which has been determined by cloning techniques, is a pentameric structure composed of xcex1, xcex2, xcex3, xcex4 and/or xcfx81 subunits. To date, 6 xcex1 subunits, 3 xcex2 subunits, 3 xcex3 subunits, 1 xcex4 subunit and 2 xcfx81 subunits have been identified and sequenced. Five of these subunits (for example 2xcex11 2xcex22 xcex32) combine together to form a channel permeable to chloride ions. By binding to this GABA-A receptor, GABA increases the permeability of the channel to chloride ions, thus inhibiting neuronal transmission. In the light of the large number of possible permutations of the various subunits, the GABA-A receptor is observed to be extremely heterogeneous in the brain of mammals and different structures in the brain generally show a preference for certain combinations of subunits.
The search for ligands which are selective for one of these various subclasses of GABA-A receptors is a major object of clinical medical research in this field.
Apart from GABA, a large number of various classes of compounds which bind to the GABA-A receptor are known. Some products, such as muscimol and isoguvacine, bind directly to the same site as GABA on the GABA-A receptor and stimulate the receptor in the same way as GABA itself. In contrast to these agonists, some substances, such as bicuculline (2), competitively inhibit the action of GABA. Such antagonists of the GABA receptor show convulsant properties in vivo (P. Krogsgaard-Larsen, B. Frolund, F. S. Jorgensen, A. Schousboe, J. Med. Chem., 1994, 37, 2489).
The inhibitory action of GABA can be modulated by compounds which interact with a variety of allosteric sites on the GABA-A receptor distinct from the GABA recognition site. One of the best known classes of allosteric modulators of the GABA-A receptor is that of the benzodiazepines (for example diazepam (3)). By thus binding to their own recognition site on the GABA-A receptor (the benzodiazepine receptor or BZR), these compounds improve the action of GABA by increasing the frequency of opening of the chloride channel (R. E. Study, J. L. Barker, Proc. Natl. Acad. Sci. USA, 1981, 78, 7180). This results in the anticonvulsant, anxiolytic, sedative-hypnotic and muscle-relaxant activities of these products, widely used clinically. Other classes of compounds structurally unrelated to benzodiazepines, such as triazolopyridazines (for example Cl 218872 (4)), imidazopyridines (for example zolpidem (5)), cyclopyrrolones (for example zopicolone (6)) and xcex2-carbolines (for example xcex2-CCM (7)), can also bind to the benzodiazepine receptors. In the case of the latter, some derivatives inhibit, rather than enhance, the neuroinhibitory action of GABA (R. L. Macdonald, R. E. Twyman in xe2x80x9cIon Channelsxe2x80x9d ed. by T. Narahashi, Vol. 3, pp. 315-343, Plenum Press, New York, 1992). In this case, the compounds, generally convulsant, are referred to as inverse agonists (or negative allosteric modulators) of the BZR, in order to distinguish them from the therapeutically useful agonists (or positive allosteric modulators) of the BZR. Some of these products show selectivity at the level of the various subclasses of GABA-A/benzodiazepine receptors. Thus, zolpidem, used clinically as hypnotic, is selective for the subclass of benzodiazepine receptors which is found predominantly in the cerebellum (BZ1 receptors) (S. Arbilla, H. Depoortere, P. George, S. Z. Langer, Naunyn-Schmiedeberg""s Arch. Pharmacol., 1985, 330, 248). This selectivity is reflected either by a narrower spectrum of activity (for example, anxiolysis without a hypnotic effect) or by a reduction in the undesirable effects of this type of product (habituation, dependency, amnesia, and the like).
Other sites exist on the GABA-A receptor which also make it possible, according to the binding of the receptor with an appropriate molecule, to modulate the activity of GABA. Mention should be made, among these sites, of those for neurosteroids (for example 3xcex1-OH-5xcex1-pregnane-20-one), barbiturates (for example pentobarbital), anesthetics (for example propofol), t-butyl-bicyclophosphorothionate cage convulsants (for example TBPS, 8), which bind to the picrotoxin site of the GABA-A receptor (W. Sieghart, Pharmacol. Rev., 1995, 47, 181 and C. R. Gardner, W. R. Tully, C. J. R. Hedgecock, Prog. Neurobiol., 1993, 40, 1). Other binding sites, less well characterized but apparently distinct, are those of loreclezole and of xcex3-butyrolactones. Such compounds also positively modulate the action of GABA and this effect is reflected by an in vivo anticonvulsant and/or anxiolytic action.
It has recently been demonstrated that gem-dialkylated xcex3-butyrolactones (9a) and gem-dialkylated xcex3-thiobutyrolactones (9b) can either reduce or enhance the action of GABA according to the position and the size of their alkyl substituents (K. D. Holland, M. G. Bouley, D. F. Covey, J. A. Ferrendelli, Brain Res., 1993, 615, 170). These compounds allosterically inhibit the binding of [S35]TBPS to rat brain membranes but do not displace [H3]-flunitrazepam from its binding site, not enhancing either the binding of benzodiazepines or of muscimol. This suggests that the compounds of type 9 may act on a different site from those already characterized on the GABA-A receptor complex.
Finally, loreclezole (10) is a novel compound which demonstrates both an anticonvulsant activity and an anxiolytic activity in various animal models (A. Wauquier et al., Drug Dev. Res., 1990, 19, 375 and G. R. Dawson, R. Curnow, P. Bayley, A. Rambridge, M. D. Tricklebank, Eur. J. Pharmacol., 1994, 252, 325). These compounds have only a negligible affinity for benzodiazepine recognition sites. Although the direct interaction of loreclezole with GABA-A receptors has been demonstrated in recombinant receptor studies, the relationship between the loreclezole binding sites and the other allosteric binding sites of this receptor is not clear to date. It has recently been demonstrated that the affinity of loreclezole for receptors comprising xcex22 or xcex23 subunits is 300 times greater than that for receptors comprising the xcex21 subunit (P. B. Wingrove, K. A. Wafford, C. Bain, P. J. Whiting, Proc. Natl. Acad. Sci. USA, 1994, 91, 4569). This selectivity may explain the absence of sedative effects of loreclezole and suggests that the compounds interacting with the loreclezole binding site on the GABA-A receptor may have important therapeutic applications.
It is therefore clear that a large number of allosteric modulatory sites, which can enhance the action of GABA and thus show a therapeutic effectiveness in a wide range of disorders of the central nervous system, exist on the GABA-A receptor. It may therefore be reasonably concluded that novel chemical structures may discover other allosteric modulatory sites currently uncharacterized on the GABA-A receptor or may bind to known sites with greater affinities or greater selectivities. Such compounds may, as a result, show a powerful and/or highly specific activity and weaker undesirable side effects in the treatment of such disorders. 
Covey et al. were the first to indicate the convulsant and anticonvulsant properties of gem-dialkyl-1,4-butyrolactones (W. E. Klunk, A. C. McKeon, D. F. Covey, J. A. Ferrendelli, Science, 1982, 217, 1040). They found that, whereas dialkyl substitutions in the xcex2 (that is to say C-3) position of butyrolactone (for example compound 54) produces convulsant effects in mice, similar substituents in the xcex1 (that is to say C-2) position (for example compound 55) lead to compounds having anticonvulsant properties in mice, preventing the attacks induced by pentylenetetrazole and picrotoxin. Structure-function studies on compound 55 indicated that the anticonvulsant activity was maintained as long as the alkyl substituents comprised 4 carbon atoms or less. If more carbon atoms were present, convulsant effects were observed. These authors also studied xcex1-alkyl substituted -1,4-thiobutyrolactones (for example 48, xcex1-EMTBL) (D. F. Covey et al., J. Med. Chem., 1991, 34, 1460), cyclopentanones 56 and lactams (for example 57). Likewise, anticonvulsant activities were observed in these three series of compounds with short-chain alkyl substituents. 
In vitro data for the most active examples of these compounds in their ability to displace S35-TBPS (indicating an affinity with the picrotoxin binding site of the GABA-A receptor) and their ability to stimulate currents produced by GABA in cultured neurons are shown in table 5.
The most active compounds of the invention (30, 31, 34, 35) are generally more powerful in displacing TBPS (see tables 2a and 2b) than the published compounds shown in table 5. More importantly, the active compounds of the invention are more powerful in stimulating the currents produced by GABA. Thus, whereas the percentage of stimulation of GABA currents by relatively high concentrations (0.3-1 mM) of the published compounds were invariably less than +200%, the compounds of the present invention stimulate the GABA response by up to +700%, this being achieved at lower concentrations (100 xcexcM).
The observation that there is very little correlation between the TBPS displacement capacities and the GABA potentiation (and with the anticonvulsant capacity) led Covey et al. to suggest that the compounds of this family (in particular xcex1-EMTBL, 48, on which the largest number of studies have been carried out) act partially by binding to a site at or close to the picrotoxin site of the GABA-A receptor, as well as to a separate site of the receptor specific to lactones and thiolactones (the butyrolactone site) (D. F. Covey et al., Mol. Pharmacol., 1997, 52, 114). It has been demonstrated that these compounds do not bind to the barbiturate and benzodiazepine binding site of the GABA-A receptor (D. F. Covey et al., Neuropharmacology, 1996, 35, 123).
On the other hand, there has been no suggestion or indication in the literature that dialkyl-1,4-butyrolactones and -thiobutyrolactones interact with the anticonvulsant loreclezole (10) recognition site of the GABA-A receptor. Indirect evidence suggests that this is not the case since xcex1-EMTBL (48) stimulates GABA-A receptors composed only of a subunits (D. F. Covey et al., Neuropharmacology, 1996, 35, 123), whereas it is known that the loreclezole binding site is present only on receptors comprising xcex22 and xcex23 subunits (P. B. Wingrove, K. A. Wafford, C. Bain, P. J. Whiting, Proc. Natl. Acad. Sci. USA, 1994, 91, 4569).
It is thus apparent that the presence of a substituted amine functional group in the C-3 position of 2,2-dialkyl-1,4-butyrolactones, a subject matter of the present invention, leads to a greater enhancement in the GABA effects than the analogous molecules described which do not have this functionality. It is also apparent that, at least in some cases, some of the compounds of the invention can act via the new loreclezole binding site on the GABA-A receptor. These two properties give the compounds of the invention a potentially high therapeutic value.
The present invention thus relates to the compounds of general formula: 
in which:
Z represents a sulfur or oxygen atom,
the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
X represents a CO, a CO2, an SO or an SO2,
and the R group represents an alkyl, aryl, alkenyl or aralkyl group,
provided that, when Z represents an oxygen atom, X an
SO2 and R a 
group,
R1 and R2 do not both represent the methyl group.
The preferred compounds of the present invention are such that Z represents an oxygen atom. This is because they have a more advantageous activity.
A specific group of compounds according to the invention is composed of the compounds of general formula: 
in which:
X represents a CO, a CO2, an SO or an SO2,
and the R group represents an alkyl, aryl, alkenyl or aralkyl group.
Mention may be made, among the preferred compounds of the invention, of the compound of formula: 
The term xe2x80x9calkyl groupxe2x80x9d is understood to denote linear or branched and substituted or unsubstituted alkyl groups comprising 1 to 8 carbon atoms. Preferred examples of alkyl groups are the CH3, CH2CH3, CH2CH2CH3 and C(CH3)3 groups.
The term xe2x80x9calkenyl groupxe2x80x9d, is understood to mean linear or branched and substituted or unsubstituted alkenyl groups comprising 1 to 6 carbon atoms. A preferred example of alkenyl groups is CH2CHxe2x95x90CH2.
The term xe2x80x9caryl groupsxe2x80x9d is understood to mean substituted or unsubstituted aromatic rings having at least 3 carbon atoms which have just one or several aromatic nucleus. The aromatic rings can be fused. Examples of preferred aromatic rings are phenyls and naphthyls. Examples of preferred substituents of these rings are alkyl groups, such as CH3, halogen atoms, such as Cl, halogenated alkyl groups, such as CF3, groups of the OCH3 type and amines, such as NH2 or N(CH3)2.
The term xe2x80x9caralkyl groupsxe2x80x9d is understood to mean aryl groups, defined as above, bonded to the CO or SO or CO2 or SO2 group via an alkyl group defined as above. A preferred example of an aralkyl group is the CH2Ph group.
The compounds according to the invention all have a center of asymmetry and can therefore exist in the form of optical isomers. The present invention equally well comprises these isomers, either separately or as a mixture.
The present invention also relates to the method of preparation of the compounds of following general formula I: 
in which:
Z represents a sulfur or oxygen atom,
the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
X represents a CO, a CO2, an SO or an SO2,
and the R group represents an alkyl, aryl, alkenyl or aralkyl group,
which can be as follows:
the amine of the compound of general formula: 
in which:
Z represents a sulfur or oxygen atom, preferably an oxygen atom,
and the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
is treated with an appropriate reactant in order to obtain the compounds according to the present invention. Preferably, this reactant is one of the family of the acyl chlorides, sulfonyl chlorides or chloroformates.
This process can comprise a preliminary stage of hydrogenolysis in order to remove the benzyl group from the compound of general formula: 
in which:
Z represents a sulfur or oxygen atom, preferably an oxygen atom,
and the R1 and R2 groups, which can be identical to or
different from one another, each represent an alkyl group or an alkenyl group.
This stage can be preceded by a stage of alkylation in the C-2 position of the compound of general formula: 
in which:
z represents a sulfur or oxygen atom, preferably an oxygen atom,
and the R1 group represents an alkyl group or an alkenyl group,
by treatment with a strong base, followed by the addition of an alkylating agent of general formula R2X in which R2 represents an alkyl, alkenyl or aralkyl group.
This stage can also be preceded by a stage of monoalkylation in the C-2 position of the 3-benzyl-aminolactone of general formula: 
in which:
Z represents a sulfur or oxygen atom, preferably an oxygen atom,
by treatment with a strong base, followed by the addition of an alkylating agent of general formula R1X in which R1 represents an alkyl, alkenyl or aralkyl group.
This stage can also be preceded by a stage of 1,4 addition of Michael type of benzylamine to an unsaturated 1,4-butyrolactone or 1,4-thiobutyrolactone to give the corresponding 3-benzylaminolactone.
In a preferred preparation process according to the present invention, the starting material which makes it possible to obtain, as in the preceding stage, the corresponding optically pure 3-benzylaminolactone is a D- or L-aspartic acid. The compounds obtained at the end of this process, after the 5 stages described above, are then optically pure.
In another preparation process example, the strong base used to carry out the alkylations is lithium hexamethyldisilazide.
The present invention also relates to pharmaceutical compositions comprising, as active principle, a compound of following general formula: 
in which:
Z represents a sulfur or oxygen atom,
the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
X represents a CO, a CO2, an SO or an SO2,
and the R group represents an alkyl, aryl, alkenyl or aralkyl group,
and an appropriate excipient. These compositions can be formulated for administration to mammals, including man. The dosage varies according to the treatment and according to the ailment in question. These compositions are produced so as to be able to be administered by the digestive or parenteral route.
In the pharmaceutical compositions of the present invention for oral, sublingual, subcutaneous, intramuscular, intravenous, transdermal, local or rectal administration, the active ingredient can be administered in unit administration forms, as a mixture with conventional pharmaceutical carriers, to animals or human beings. The appropriate unit administration forms comprise oral forms, such as tablets, gelatin capsules, powders, granules and oral solutions or suspensions, sublingual and buccal administration forms, subcutaneous, intramuscular, intravenous, intranasal or intraocular administration forms and rectal administration forms.
When a solid composition is prepared in the form of tablets, the main active ingredient is mixed with a pharmaceutical vehicle, such as gelatin, starch, lactose, magnesium stearate, talc, gum arabic or similar materials. The tablets can be coated with sucrose or with other appropriate materials or they can be treated so that they have a prolonged or delayed activity and so that they continuously release a predetermined amount of active principle.
A preparation in gelatin capsules is obtained by mixing the active ingredient with a diluent and by pouring the mixture obtained into soft or hard gelatin capsules.
A preparation in the syrup or elixir form can comprise the active ingredient in conjunction with a sweetener, an antiseptic, and a flavor enhancer and an appropriate dye.
The water-dispersible powders or granules can comprise the active ingredient as a mixture with dispersing agents or wetting agents, or suspending agents, and with flavor enhancers or sweeteners.
For rectal administration, recourse is had to suppositories which are prepared with binders which melt at the rectal temperature, for example cocoa butter or polyethylene glycols.
For parenteral, intranasal or intraocular administration, use is made of aqueous suspensions, isotonic saline solutions or sterile injectable solutions which comprise pharmacologically compatible dispersing agents and/or wetting agents.
The active principle can also be formulated in the form of microcapsules, optionally with one or more additional carriers.
The present invention also relates to the use of the compounds of following general formula: 
in which:
Z represents a sulfur or oxygen atom,
the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
X represents a CO, a CO2, an SO or an SO2, and the R group represents an alkyl, aryl, alkenyl or aralkyl group,
as stimulant of the activity of xcex3-aminobutyric acid acting via the GABA-A receptor of the central nervous system.
The compounds according to the invention of following general formula: 
in which:
Z represents a sulfur or oxygen atom, the R1 and R2 groups, which can be identical to or different from one another, each represent an alkyl group or an alkenyl group,
X represents a CO, a CO2, an SO or an SO2,
and the R group represents an alkyl, aryl, alkenyl or aralkyl group,
and the pharmaceutical compositions comprising them can be used as a medicament, in particular for the treatment of nervous disorders. These disorders are preferably of the following types: epilepsy, anxiety, depression, sleep disorders, panic attacks, muscular contractions, pain, dependence on alcohol or benzodiazepines, or psychotic behaviors.